This invention relates to a metal backed printed circuit board assembly having an improved conductive interconnection between the printed circuit board and the metal back plate thereof.
In many applications, such as cellular systems, personal communication systems, and land-based communication systems, printed circuit boards are connected to metal back plates to provide a metal-backed printed circuit board assembly. The metal back plate may be bonded to one surface of a thin, flexible, printed circuit board to rigidify the board and thus, make the resulting assembly easier to handle. The metal back plate may also act as a sink for dissipating the heat that is generated when additional components are soldered to the opposite circuitized surface of the printed circuit board, thereby preventing damage to the circuitry. In some applications the metal plate also acts as an electrical ground plane.
Several methods have been used to connect the metal back plate to the printed circuit card. One method involves soldering the printed circuit board onto the metal back plate. This method provides a conductive interconnection between the entire adjoining surfaces of the printed circuit board and the metal back plate. Unfortunately, the soldering process can cause flux to become entrapped between the metal back plate and the printed circuit board. Since flux is corrosive, it will eventually degrade the conductive interconnection. Sweat soldering is difficult to achieve without entrapping air and creating voids in the conductive interconnection. The presence of voids disrupts localized grounding, and in some cases can degrade the system""s electrical performance. Sweat soldering also subjects the printed circuit board to significant thermal stress, which can increase fatigue of critical structures in the board and decrease product lifetime. Sweat soldering is also a costly manufacturing process. Moreover, the soldered bond is relatively non-compliant. In addition, the assembly tends to develop defects due to CTE mismatches during subsequent processing or use, thus rendering the mechanical and electrical connections between the printed circuit board and metal back plate unreliable and prone to failure.
Simple mechanical interconnections such as screws or rivets may also be used to connect the printed circuit board to the metal back plate. However, imperfect coplanarity between the printed circuit board and the metal back plate coupled with localized thermal stresses tend to cause development of localized areas of non-contact between the printed circuit board and metal back plate. Accordingly, the interconnection between the printed circuit board and the plate does not remain global and can, in fact, vary significantly with time. Moreover, the resistance across the interface between the printed circuit board and the metal plate tends to increase over time, which may lead to failure of the assembly.
Another method employs a conductive adhesive which is heavily loaded with conductive particles, particularly silver particles, thereby creating conductive paths that carry current from the printed circuit board to the metal back plate. The printed circuit board assemblies made with some of these adhesives initially have a low bulk resistivity. Unfortunately, however, the metal back plate of such adhesively-bonded assemblies tend to corrode, readily under conditions of high humidity and elevated temperatures. This problem is especially troublesome when the metal back plate is made of aluminum and the conductive adhesive contains a non-noble metal, such as silver. The corrosive products that are formed at the interface between the conductive adhesive and the metal back plate ultimately can lead to electrical and mechanical instability and failure of the bonding between the printed circuit board and the metal back plate.
Accordingly, it is desirable to have an improved method of conductively bonding a printed circuit board to a metal back plate. A method that produces a printed circuit board assembly in which the mechanical and electrical connections between the printed circuit board and the metal back plate remain stable even when the assembly is subjected to humid environments over extended periods of time is especially desirable
In accordance with the present invention, a method for conductively bonding a printed circuit board to a metal back plate is provided. The method comprises providing a dielectric substrate that is metallized on its two faces; providing a metallic back plate; and bonding the metallic back plate to one of the metallized faces of the substrate using an electrically conductive adhesive that comprises a polymeric adhesive and at least one conductive metal having an electromotive force (EMF) that is less than one (1) volt. Preferably, the adhesive is substantially free of conductive metals having an EMF of 1.0 volt or greater.
The present invention also relates to a printed circuit board assembly comprising a printed circuit board comprising a dielectric substrate having a first circuitized metallic layer disposed on one opposing face of the substrate and a second metallic layer disposed on the other opposing face of said substrate; a metal back plate; and an electrically conductive bonding layer disposed between the plate and the second metallic layer of the printed circuit board. The electrically conductive bonding layer comprises a polymeric adhesive and a conductive metal having an EMF that is less than one volt. The electrically conductive bonding layer is substantially free of silver.
In accordance with the present invention, it has been discovered that employing a conductive adhesive that comprises conductive metal particles having an EMF of less than one volt to bond a printed circuit board to a metal back plate can reduce or even eliminate the increase in electrical resistance that occurs at the interface between the adhesive and the metal plate when the printed board assembly is subjected to temperature fluctuations and/or humid conditions.